I think 255 might be a bit high for a few reasons. First, that 20 min interval is right at the beginning of the ride - meaning the rider's full anaerobic capacity can be used to generate power. I also see a dip in HR towards the end, implying a drop off in power generation, although it's hard to tell from a screen cap from Garmin's site as there's not enough detail. Terrain and/or traffic could have been an issue, but that HR dip does imply that the early parts of the interval were overdone. Third, outside of the final burst, power levels look like they trend downward over the course of the interval.

All those are consistent with anaerobic power generation adding to aerobic power. But we're trying to measure purely aerobic power.

I'd call it 245W if I had base it off just this one ride. But again, it's only one 20-minute interval. I've done FTP tests where I couldn't hold 90% of an FTP value that was known good for 10 minutes, much less 105% for 20. Some days just suck.